A rapid and simple method for determining the proviral DNA content in peripheral blood mononuclear cells (PBM) from patients with human immunodeficiency virus type I (HIV-1) infection was established by using the polymerase chain reaction (PCR) technique. We found that the majority of HIV proviral DNA copies detectable in unfractionated PBM resided in T cells, while B cells/monocytes contained lesser amounts of HIV DNA (93.9+- 3.5% for T cells versus 6.1+-3.5% for B cells/monocytes: p<0.05). When we compared the amount of HIV proviral DNA in PBM from 13 patients with AIDS or ARC before and during antiretroviral therapy with 2',3'dideoxyinosine (ddI) which was given as an escalating dose in a phase I clinical study, a significant decrease was observed in 9 out of 12 evaluable patients receiving the drug for 8 to 14 weeks (p<0.02). The decrease appeared more pronounced in patients receiving relatively high doses of the drug. These data suggest that the quantitation of HIV viral DNA in PBM by PCR is feasible and may theoretically contribute to an overall monitoring of patients receiving experimental therapy. However, larger studies will be required to determine the sensitivity and specificity of this assay and further longitudinal studies will be essential.